Halogenation of Alkanes
5-4

5-5

5-6 Radical bromination is more selective because of its slightly higher activation energy required to break a C-H bond during the propagation steps (when the bromine radical abstracts a proton from the substrate). Though the difference in activation energy is not huge (Cl = ~1 kcal/mol and Br = ~3 kcal/mol), it leads to a significant difference in selectivity.